Simple Renewables www.simplerenewables.com The Wind Trubine The Wind Turbine Generator (WTG) is the machine capable of turning wind into electricity, there are thousands of WTG's of different sizes deployed around the globe and most of them share a particular set of characteristics. This paper illustrates the main components of a WTG, and how they work in order to convert an air ow into usable electricity, in addition of a brief introduction of the main principles that make this technology possible, considering some of the main constrains of the technology. Designed and created by César Sierra csierra@simplerenewables.com First Published in 14/04/2021 Version 1.1 Last updated in 21/04/2021 Simple Renewables www.simplerenewables.com Wind Rotor The wind that acts on the blade airfoils is composed from the natural ow of air and the ow of air produced by the rotation of the blades The further away from the hub the greater the rotational component of the wind Wind turbine blades are composed of multiple airfoils Hub Nacelle The nacelle and the blades continuously align themselves to maximize the ow of air through the blades Perpendicular to the ow of air is the lift Blade Parallel to the ow of air is the drag An airfoil is solid geometrical shape that generate aerodynamic forces Tower Modern onshore turbines have hub heights around 120m and continue to rise each year Boeing 747-8 Boeing 747-8 has a length of 76.4m www.simplerenewables.com Simple Renewables www.simplerenewables.com Power output and efciency An oversimplication of how WTG generate energy, is partially given by the power formula which is derived from the kinetic energy formula. However, the turbine components efciency must be considered to obtain the nal power output. The power formula is dependent of 3 main things, rotor diameter, air density and wind speed. 1 m v2 E = __ 2 Wind turbine blades extract energy form wind particles crossing the swept area, the particles that hit the blades transmit energy by reducing their speed, since their mass is constant. v m m E P = ___ t _1 mv2 2 P = ___ t d t =v Wind speed affects the power output more than any other factor, the derivation of the mas ow and kinetic energy result in a cubic wind speed. V = A1 A The swept area is given by the length of the blades and it is the main mechanical improvement that increase the power of a turbine. 1meter 1 m v2 E = __ 2 m = Vρ m = A1ρ m = Mass ow = V ρ v __ t m = A1ρ v __ t 2 1 P = _ A1ρ v v 2 3 1 P = _ A ρv 2 altitude temperature atmospheric pressure ρair humidity The air density is dependent of multiple factors, the altitude and temperature are the most inuential. 1 __ 6 2 __ 6 3 __ 6 vinitial = 1 4 __ 6 Initial speed 5 __ 6 vnal = 1/3 Final speed 6 __ 6 3V __ 3 Theoretical energy extracted Not all wind is affected by the WTG and not all energy can be extracted from the wind 2 2 2V x __ 88% = 59.26% Betz limit 3 = (1) - (1/3) = 88.88% 2 2 = vinitial - vnal The most optimal WTG would reduce 2/3 of the wind speed, from 2/3 of the air volume passing through the swept area, this is the theoretical limit of energy that a turbine can extract form the wind. Modern turbines efciency Generator Other components WTGefciency Gearbox Blade design www.simplerenewables.com 30% - 50% 3 1 _ WTGefciency A ρv P= 2 Simple Renewables www.simplerenewables.com Blade design Faster, longer or thicker, Ideal rotors should be allowed to spin faster and with long blades that cut through air with minimum drag, nevertheless these traits come with their constrains. Modern turbines are optimized to generate as much torque possible within the possibilities of their mechanical and environmental constrains. Depending on the wind speeds, rotational speeds or angle of attack, airfoils work differently, ideally they generate the maximum lift possible for the lowest drag possible Lift vs Drag Slim airfoils are optimal for high-speed rotations generating low drags Blade Length The sweet spot lies in blades that combine multiple airfoils according to their wind speeds Thicker airfoils generate higher lifts even in low speeds although they tend to have large drags The main constrain of longer blades come with the materials stiffness vs cost, long elastic blades could hit the WTG tower in high winds Thicker airfoils require additional structure to maintain their form while slimer airfoils may not have enough space for structure and require expensive materials to compensate Longer blades generate more torque as the lift momentum increases with the length Rotational speed High rotational speeds are ideal as blades are able to extract more energy from the wind and even generate less turbulence, however this comes with an increase on mechanical stress and heavy noise Fast rotating blades disrupt air ow less but can extract more energy from it www.simplerenewables.com Naturally turbines want to rotate faster, similar to a pinwheel, the faster you blow the faster they rotate Simple Renewables www.simplerenewables.com The acting forces Due to their geometry and orientation the lift and drag forces decompose into different components regarding the WTG's orientation, in the end two forces result, torque and thrust, the rst one rotates the generator, the second one is “absorbed” by the structural components of the turbine. 3 different planes act over the airfoil, the direction of the wind, the direction of the airfoil and the direction of the rotation The lift and drag components on the rotation plane are the ones that affect the energy output of the WTG The difference between wind direction and airfoil direction is called angle of attack, this is controlled by the pitch of the blades Wind ow The lift and drag component on the rotation plane will result on 3 forces, a lift component, drag component and a thrust component, the rst two will subtract resulting on a nal acting lift that contributes to the torque Lift Theory The same way as an airplane air molecules passing through a blade create different pressure zones that generate the lift Rotation air ow LIFT Natural air ow + = DRAG Both lift and drag component will add on the thrust plane, the thrust is the force trying to tip over the WTG Rotation plane Wind direction Airfoil direction L The further away from the hub a lifting force acts, the greater the torque force it will generate, on the same way the thrust forces will act on the turbine structure L www.simplerenewables.com Yaw and Pitch control Simple Renewables www.simplerenewables.com Yaw and pitch control are essential for an optimal energy production, any misalignment in the blades angle of attack (pitch) or in the turbines direction towards the wind ow (yaw) will result in a diminish of torque in addition to unwanted stress in the structural components. WTG's have systems that continuously change yaw and pitch to optimize production Near the blade tip, the air ow produced by the rotation is higher than the natural air ow The correct angle of attack will maintain the best lift vs drag relation, maximizing torque and minimizing thrust Blades are twisted along their length, this way the entire blade is oriented towards its ideal angle Near the hub rotational air ow is minimum, but the natural ow maintains the same speed through the blade, the perceived wind ow direction changes Pitch If the blade is under pitched it will generate a low amount of Torque and Thrust, this can be useful to slow down the blades or to stand in idle position This is a common distribution of Torque and Thrust forces along the blade Yaw If the WTG is not aligned correctly towards the incoming wind (yaw), the blades will perceive wind differently in each stage of the rotation, this will diminish their production and produce unwanted structural loads www.simplerenewables.com When the blades are over pitched they will generate an immense amount of force, however the drag component will diminish the additional lift Simple Renewables www.simplerenewables.com Main Components What's inside the box? Most modern WTG's share similar characteristics, the size, location and layout varies among manufacturers but most modern turbines look alike, most components are located inside the nacelle but some can also be found at the bottom of the tower or even outside the turbine Control system Control cables Gearbox Low speed rotor Power electronics Holding disk brake Generator High speed rotor Electric cables Condition monitoring sensors Ventilation system Rotor Bearings Cooling system Aviation lights Access hatch Main inner structure Fire suppression system Nacelle MET sensors Hub Pitch control motors Outer vents Yaw control motors Access shaft Service elevator Tower www.simplerenewables.com Simple Renewables www.simplerenewables.com Power Curve The mind and soul of the WTG lies in its generating system, the controller unit which acts as the brain, uses a network of sensors to monitor and orchestrate the functionality of the main components, this allows WTG to convert variable wind speed into reliable and usable high voltage alternating electric current. The nal power output of the WTG is plotted in a graph called the power curve, this graph demonstrates de capabilities of the system for a different range of conditions, the power curve is essential for the development and commissioning of wind farms. Gearbox Controller & sensors Most WTG use a single stage planetary gearbox, that transform the low speed rotation of the blades into the high speed rotation needed by the generator A continuous monitoring system is in the core of the controller, measuring rotation speeds, wind speeds, vibrations, temperatures, power outputs, etc, processing al the data inputs and adjusting the WTG as needed Variable alternating current Direct current Synchronized alternating current Doble fed asynchronous generator Power Electronics Wind turbine generators differ form conventional generators as they are asynchronous, since the rotational speed that feeds them is not constant, power electronics are used to compensate by feeding one of the windings (Stator / rotor) with alternative current, that constantly variates in phase and frequency, these variations are calculated to compensate with the rotational variations, therefore the other set of windings produces synchronous alternating current (50/60Hz) The initial part of the curve is given by the power formula Power MW 4.5 4.0 3.5 3.0 3 1 P = _ A ρv WTGefciency 2 The system is feed from the output AC, which is then converted to DC, and then converted back to AC in the frequency and phase needed by the generator The WTG constrains and capabilities limit and stabilize the power output into its rated power Rated power This curve represent how blades would stall and lose efciency if it wasn't for pitch control 2.5 2.0 1.5 1.0 0.5 4 2 Cut in speed The minimum wind speed for operation 6 8 10 Rated speed 16 14 12 wind speed m/s The minimum speed needed to achieve rated power www.simplerenewables.com 18 20 26 22 24 Re-cut in speed Cut out speed The average speed needed to restart operation The maximum speed for safety operation Simple Renewables www.simplerenewables.com Structure New materials have made possible for turbines to keep growing, but the structural design of WTG is a quite a complex subject, as modern turbines are massive moving objects subjected to changing forces due to the variability of forces, turbines must withstand the constant changes without losing efciency or having major wear on its components. The Thrust force on the blades will cause a momentum on the rotor, most of it will cancel out by the momentum of the other blades The sum of all the components weight will be added to the momentum forces created by the Thrust, vibrations, and air drag over the entire turbine Any variation on the position, wind speed, misalignment or unbalance on the blades will create additional momentum forces that will be transmitted through the rotor and create vibrations The Thrust and drag will create a sheer force through the tower Wind shear is an important issue as it can cause a mismatch on the forces, as the top blade will have more thrust than the lower ones The nal result of all the forces at the bottom of the tower will vary since the air speed, direction and blade positions are constantly changing All the added forces will be transmitted through the tower to the foundation which will dissipate them into the ground, the soil need to have the necessary strength to manage this forces www.simplerenewables.com Simple Renewables www.simplerenewables.com Wind Farm development & construction Most of this section is a working progress, keep checking www.simplerenewables.com for any updates. Designed and created by César Sierra csierra@simplerenewables.com Simple Renewables www.simplerenewables.com The Wind Measurement campaign The wind measurement is an essential although lengthy process that any large scale wind energy project must undertake, a successful campaign will result with the selection of an ideal site and the optimal turbine for that specic site, in addition a successful campaign will result in a precise production forecast that in the end will provide a nancially healthy project. Designed and created by César Sierra csierra@simplerenewables.com First Published in 14/10/2020 Version 1.3 Last updated in 12/04/2021 Simple Renewables www.simplerenewables.com Site Selection for wind projects What is the rst step in developing a wind project? In most cases it will be the collection of multiple information sources related with the needs of the project, a demand for energy, a way to transmit power, available land and most important windy areas (resource) a Geographical Information System is the perfect way to combine and visualize multiple layers of information in order to highlight essential information. High resolution wind maps are a combination of metadata obtained of multiple sources and processed to show the fundamental information. Transmission map + Roads, urban areas & infrastructure projects Transmission voltage Projected grid works Consumption areas Line saturation + Constrains map Geographical limitations Protected areas Possible natural disasters Resource map Satellite resource data Nearby site measurements Usual error is between 2.0 and 0.5 m/s Archaeological sites GIS can integrate multiple and complex spatial data with the goal of highlighting areas of interest. Land availability Environmental impact Social Impact (Geographic information system) GIS Map Site access Potential consumers Once all the layers are added a regional, detailed, high resolution map is extracted, showing the main areas of interest Existing wind farms Available infrastructure Areas of interest where site scouting will be done, are marked on the GIS map Existing MET stations High resource area Wind farms Intermediate resource area Protected areas Urban areas Low resource area Local constrains Sub-stations www.simplerenewables.com Conventional power plants Power lines Power consumers Simple Renewables www.simplerenewables.com The MET station The Meteorological station is the backbone of a wind development project and essential for lowering the risk taken to develop and nance a wind project. It's important to know Where to install the station? and What to install on them? For the rst question there is a series of methods to determine the ideal spot, regional wind maps & topographical data can be simulated to nd ideal spots, and satellite images help identify local constrains, however a site visit is always useful as there are always accessibility and geographical issues that affect. For the second question I say the more the merrier, adding a few more sensors will increase the station precision and reliability while only increasing a fraction of the overall cost of the station. Met stations are still key elements in the development and operation of wind farms but new technologies are looking to improve their performance or maybe even replace them. Redundancy is key, multiple sensors help correlate the data or as back up for malfunctions Current met towers can't keep up with the increasing heights of WTG, multiple measurement methods help achieve a complete assessment Having different sensor models increase redundancy The anemometer measures horizontal wind speed, there are multiple classes that determine precision and reliability 120m Hub height Wind vanes measure horizontal wind direction, its critical to align and orient them correctly 100m Maximum cord Temperature, pressure, and humidity will directly affect the density of the air and the amount of energy produced 80m The vertical wind speed helps correct measurement errors, wind true angle of attack & turbulence 60m Tip of the blade Multiple height measurements help calculate the wind gradients or wind shear The sensor layout is usually designed for a specic WTG 011001 7.58 The datalogger decodes and records all the sensor signals it also stores and communicates the data A power unit and back up batteries keep the station running 24/7 Stations should be located where the measurements are the most representative Stations have a sphere of inuence that can be limited by geographical features A preliminary layout helps select the correct spots www.simplerenewables.com Site accessibility and site constrains inuence the selection of the site Simple Renewables www.simplerenewables.com SODAR’ & LIDAR’ What will we do now that new turbine heights are unreachable for met masts? The solution may lie on new technologies such as SODAR's and LIDAR's that are now allowing us to complement the measurement campaigns, study the behavior of existing WTG's and even reach out into new sites that previously where to difcult or expensive to access. SODAR and LIDAR technologies are becoming increasingly widespread mainly due to the multiple applications and life span of a single unit, adding to this, is the cost reduction that these systems have had over the last years. Outgoing wind separates waves Stationary air bounces the same wavelength Incoming wind pushes waves together Doppler Effect The LIDAR works by beaming a laser array and detecting differences in laser return times and wavelengths Both systems are easy to transport and require a remote energy source +200m LIDAR (Light detection and ranging) The SODAR bounces high pitch sound on the wind layers, and detects the differences on wavelengths that bounce back due to air pressure differences SODAR Met mast height extrapolation & calibration SODAR's & LIDAR's (Sonic detection and ranging) Wind speed 3D direction & turbulence The result is a wind prole at multiple altitudes Power curve measurements Performance optimization Nacelle mounted LIDAR Crane safety monitoring Turbine alignment Supports MET mast by lling in the gaps on a site survey Can be airlifted to complex sites www.simplerenewables.com New site exploration Offshore measurements for monitoring or prospecting Simple Renewables www.simplerenewables.com MET Data anlysis On average a MET station generates 6 packages of information every hour, each package contains the 10min average reading of about 10 to 15 sensors, when we start to add up the amount of information that a group of stations generate on a year it can become to much to handle. Fortunately automatization and new data management technologies enable us to be manage “big data”, and new algorithms also show us analyzed data and reports sometimes in real time, now a days most resource analysis charts come from automatized reports, however it is always convenient to know how to process all this raw data by yourself. Each sensor emits a specic signal that the logger interpretates the RAW data 10min averages are widely used, shorter averages increase information but require better data handling 011001 7.58 Each met station has an average data acquisition, ideally over 95% if the acquisition is poor, maintenance must be done Loggers compress, encrypt and send the information in bundles that contain the average measurements If a station stops communicating a physical download of the data can be done by a technician A server receives, organizes and store all the unprocessed data The main outcomes of the analysis Erroneous readings or abnormal weather Wind Rose, with wind speed and frequency Vav Average Wind speed Specialized softwares can manage multiple stations, process the data and report problems m/s Daily wind prole m/s Annual wind speed prole m/s Wind speed distribution Vav Vav Vav Important factors when selecting a WTG Maximum wind gusts & turbulence Wind classes for WTG IEC 61400 Operating temperature range Sustained gust registered In the last 50 years Maximum gust registered in the last 50Yrs Average wind speed Turbulence is the amount of times wind changes speed or direction in 10min IA m/ Vav s I 10 II 8.5 III 7.5 A IIB IIIB Vgust Vref 70 60 53 50 43 38 B C +16% 14% 12% www.simplerenewables.com High humidity and low freezing temperatures cause icing on blades Simple Renewables www.simplerenewables.com Layout design This section is a working progress, keep checking www.simplerenewables.com for any updates. Designed and created by César Sierra csierra@simplerenewables.com